There are used silicon-based optical communication devices having an optical fiber communication wavelength of 1330 or 1500 nm for various systems such as home optical fibers and local area networks (LANs). The silicon-based optical communication device is a very promising technique that allows to integrate optical functional elements and electronic circuits on a silicon platform using the CMOS technology.
In recent years, silicon-based passive devices such as a waveguide, an optical coupler, and a wavelength filter have been studied very widely. In addition, silicon-based active elements such as an optical modulator and an optical switch have received a great deal of attention as an important technique of operating optical signals for such communication systems. However, an optical switch or a modulation element that changes the refractive index using the thermooptic effect of silicon operates at a low speed and is applicable to only a device speed up to a modulation frequency of 1 Mb/sec. Hence, to implement a high modulation frequency required in more optical communication systems, an optical modulation element using the electrooptic effect is important.
Many of currently proposed electrooptic modulators change the free carrier density in the silicon layer using the carrier plasma effect and thus change the real part and the imaginary part of the refractive index, thereby changing the phase or intensity of light (literature 1: Japanese Patent Laid-Open No. 2006-515082, literature 2: Japanese Patent Laid-Open No. 2002-540469, and literature 3: Japanese Patent Laid-Open No. 9-503869). Pure silicon exhibits no linear electrooptic effect (Pockels) and a very small change in the refractive index by the “Franz-Keldysh” effect or the “Kerr” effect. For this reason, silicon-based modulators widely use the above-described carrier plasma effect. A modulator using free carrier absorption directly modulates the output by changing absorption of light propagating through silicon. The structure using the refractive index change generally uses a Mach-Zehnder interferometer. The Mach-Zehnder interferometer obtains an intensity-modulated signal of light by optical phase interference between two arms.
The free carrier density of the electrooptic modulator can be changed by injecting, storing, removing, or inverting free carriers. However, many of the devices examined so far are poor in the optical modulation efficiency and require a length of mm order for optical phase modulation and an injection current density higher than 1 kA/cm3. To implement a smaller size, higher degree of integration, and lower power consumption, a device structure capable of obtaining a high optical modulation efficiency is necessary. If a high optical modulation efficiency is obtained, the optical phase modulation length can be shorter. A large device is readily affected by the temperature distribution on the silicon platform. For this reason, the refractive index change in the silicon layer caused by the thermooptic effect may problematically cancel the original electrooptic effect.
FIG. 12 shows a typical example of a silicon-based electrooptic phase modulator using a rib-shaped waveguide manufactured using an SOI (Silicon on Insulator) substrate. This electrooptic phase modulator uses, as the lower cladding, a buried insulating layer 1202 formed on a support substrate 1201 that forms the SOI substrate. A rib-shaped core 1205 and slab regions 1204 are formed in an SOI layer 1203 of the SOI substrate. Hence, the core 1205 and the slab regions 1204 are intrinsic semiconductor regions. The electrooptic phase modulator also includes a p+-type region 1206 and an n+-type region 1207, which are formed by doping the slab regions 1204 laterally extending on both sides of the core 1205 with a p-type impurity and an n-type impurity, respectively.
The structure shown in FIG. 12 forms a p-i-n diode modulator. When applied with forward and reverse biases, the structure changes the free carrier density in the intrinsic semiconductor regions such as the core 1205 and thus changes the refractive index using the carrier plasma effect.
In this example, one slab region 1204 made of intrinsic semiconductor silicon is formed so as to include the p+-type region 1206 formed by heavily doping a region in contact with a first electrode contact layer 1208 with a p-type impurity. The other slab region 1204 includes the n+-type region 1207 formed by heavily doping a region in contact with a second electrode contact layer 1209 with an n-type impurity.
In this p-i-n diode structure, the p+-type region 1206 and the n+-type region 1207 may be formed by doping so as to exhibit a carrier density of about 1020 for every cm3. In the above-described p-i-n structure, the p+-type region 1206 and the n+-type region 1207 are arranged at intervals (slab regions 1204) on both sides of the core 1205 of the intrinsic semiconductor. The core 1205 is covered with an upper cladding 1210 made of silicon oxide, thereby forming an optical waveguide.
The optical modulation operation of the electrooptic phase modulator will be described. A forward bias is applied to the p-i-n diode using the first electrode contact layer 1208 and the second electrode contact layer 1209. The modulator is connected to a power supply so as to inject free carriers into the optical waveguide formed from the core 1205 upon bias application. The bias application increases the number of free carriers and thus changes the refractive index of the core 1205, thereby phase-modulating the light transmitted through the waveguide.
Literature 1 describes a silicon-based electrooptic modulator having an SIS (Silicon-Insulator-Silicon) structure. As shown in FIG. 13, this electrooptic modulator includes a support substrate 1301 that forms an SOI substrate, a p-type main body region formed on a buried insulating layer 1302, and an n-type gate region stacked so as to partially overlap the p-type main body region. The p-type main body region includes a p-type silicon layer 1303 doped with a p-type impurity, a p+-type silicon layer 1304 heavily doped with a p-type impurity, and an electrode contact layer 1305.
The n-type gate region includes an n-type silicon layer 1307 doped with an n-type impurity, an n+-type silicon layer 1308 heavily doped with an n-type impurity, and an electrode contact layer 1309. The electrode contact layers 1305 and 1309 are, for example, a metal silicide. A dielectric layer 1306 is formed in the region where the p-type silicon layer 1303 (main body region) and the n-type silicon layer 1307 (gate region) overlap. A cladding layer 1311 made of silicon oxide is formed to cover the main body region and the gate region.
The impurity concentrations in the p-type silicon layer 1303 and the n-type silicon layer 1307 are set such that the carrier density change is controlled by an external signal voltage. In the p-type silicon layer 1303 and the n-type silicon layer 1307 on both sides of the dielectric layer 1306, optical phase modulation is performed by removing, or inverting the free carriers. Ideally, the optical signal field and the carrier density are preferably externally dynamically controlled in the same region.